1. Field of the Invention
The present invention generally relates to signal chains for image sensors, and more particularly to a mixed-signal operation for a black level compensation (BLC) and high-gain signal chain.
2. Description of Related Art
Semiconductor based image sensors such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) image sensors are widely used, for example, in cameras and camcorders, to convert images of visible light into electronic signals.
Due to imperfections in electronic circuitry, leakage current exists even when no light is received by the image sensor. In order to overcome this problem, black level signals are read from rows of light-shielded or optically black pixels and then averaged as an optical black reference to facilitate black level compensation (BLC).
FIG. 1 shows a signal chain 1 coupled to receive and amplify a signal, e.g., a black level signal, output from an image sensor (not shown) using an arrangement such as disclosed in the above noted U.S. application Ser. No. 12/477,899. According to the arrangement, in which like elements are identified with like reference numbers, a readout amplifier, e.g., a programmable gain amplifier (PGA) 12, receives the signal and provides an output, which is clamped to the reference voltage at which a black level, e.g., zero level, of the image sensor is defined. A black level compensation (BLC) circuit 10 is used in BLC mode to compensate the PGA 12, such that the formed loop (loop-1) accumulates offset value to fit the zero level of a compensation amplifier, e.g., an analog-to-digital converter (ADC) 14. A digital gain circuit 16 with variable digital gain is also included in the signal chain 1.
According to the signal chain 1 shown in FIG. 1, the final output out of the digital gain circuit 16 approaches zero level in the BLC mode. This zero-level black, however, is not optimally realistic and naturalistic to (e.g., as perceived by) the human eye. Further, the zero level may (e.g., is likely to) become saturated when dark noise is gathered. Moreover, real-world ADCs 14 can have unwanted ADC circuit offsets due to circuitry imperfections and process variations. These problems can become worse or even uncontrollable in the context of a high-gain signal chain 1 with a high-gain PGA 12 and/or digital gain circuit 16.
For the foregoing reasons, a need has arisen to propose a novel signal chain for image sensors in order to obtain a black output that is realistic or naturalistic to the human eye. Further, the novel signal chain should be capable of canceling unwanted ADC circuit offset, particularly in the context of high-gain signal chains.